Extension of the Renormalized Coupled-Cluster Methods Exploiting Left Eigenstates of the Similarity-Transformed Hamiltonian to Open-Shell Systems:  A Benchmark Study

Abstract

The recently formulated completely renormalized coupled-cluster method with singles, doubles, and noniterative triples, exploiting the biorthogonal form of the method of moments of coupled-cluster equations (Piecuch, P.; Włoch, M. J. Chem. Phys. 2005, 123, 224105; Piecuch, P.; Włoch, M.; Gour, J. R.; Kinal, A. Chem. Phys. Lett. 2006, 418, 467), termed CR-CC(2,3), is extended to open-shell systems. Test calculations for bond breaking in the OH radical and the F2+ ion and singlet-triplet gaps in the CH2, HHeH, and (HFH)- biradical systems indicate that the CR-CC(2,3) approach employing the restricted open-shell Hartree--Fock (ROHF) reference is significantly more accurate than the widely used CCSD(T) method and other noniterative triples coupled-cluster approximations without making the calculations substantially more expensive. A few molecular examples, including the activation energies of the C2H4 + H --> C2H5 forward and reverse reactions and the triplet states of the CH2 and H2Si2O2 biradicals, are used to show that the dependence of the ROHF-based CR-CC(2,3) energies on the method of canonicalization of the ROHF orbitals is, for all practical purposes, negligible.

A comparative assessment of the perturbative and renormalized coupled cluster theories with a noniterative treatment of triple excitations for thermochemical kinetics, including a study of basis set and core correlation effects

“…6 and three approximate variants of CR-CC (2,3) which are obtained by dropping terms in the full CR-CC(2,3) energy formula. 6,8 Two of these variants are practically identical to the triples corrections of the CCSD(2) theories developed in Refs. 9 and 10, giving us an opportunity to systematically examine the consistency among a larger variety of the non-iterative triples coupled cluster methods in the context of the activation barrier calculations.…”

“…The details of the more recent CR-CC (2,3) theory can be found elsewhere as well. 6,8 Here, we only mention that in analogy to the conventional CCSD(T) approach, in the CR-CC(2,3) calculations we add a correction due to triply excited clusters to the CCSD energy. The difference between CCSD(T) and CR-CC (2,3) lies in the definition of the connected triples correction, which in the CR-CC (2,3) case uses the complete form of the triply excited moments of the CCSD equations (projections of the CCSD equations on triply excited determinants) rather than the leading contributions to these moments used in CCSD(T).…”

Benchmark of Dynamic Electron Correlation Models for Seniority-Zero Wave Functions and Their Application to Thermochemistry

“…Among the most representative examples are the CR-CC (Completely Renormalized Coupled Cluster) and CR-EOMCC (Completely Renormalized Equation of Motion Coupled Cluster) approaches [18][19][20][21][22], the active-space CC and EOMCC methods [23][24][25][26], the EA/IP-(Electron Affinity/Ionization Potential) and DEA/DIP-(Double EA/IP) EOMCC models [27], and the spin-flip CC/EOMCC formalism [28][29][30]. A different, computationally feasible approach suitable for strongly-correlated systems uses seniority-zero wavefunctions to describe the static/nondynamic part of the electron correlation en-ergy.…”

SPIN-FORBIDDEN AND SPIN-ALLOWED CYCLOPROPENONE (c-H2C3O) FORMATION IN INTERSTELLAR MEDIUM

“…The energies, equilibrium geometries, MECP, and partition functions were first determined using the B3LYP density functional theory (Becke 1993;Stephens et al 1994;Hertwig & Koch 1997) and, if practical, refined with the completely renormalized coupled cluster CR-CCL method (Piecuch & Włoch 2005;Kinal & Piecuch 2007;Włoch et al 2007). If the wave function of the system had significant multireference character, as in reaction (1), the Complete Active Space SelfConsistent Field (CASSCF) method (Roos et al 1987;Ivanic & Ruedenberg 2001) was used, followed by multireference perturbation theory MRMP2 (Kozlowski et al 1995;Rintelman et al 2005;Ten-no 2007) and Multireference Configuration Interaction (MRCI) with singles and doubles method (Woywod et al 1994;Szalay & Bartlett 1995;Werner & Knowles 1988).…”